Asphaltic roofing membranes, also known as bituminous membranes, have been employed to cover flat or low-sloped roofs. These membranes are typically installed by unrolling a roll of material on a roof surface and then heat seaming adjacent membranes together to form an impervious water barrier on the roof surface.
As part of the manufacturing process, the asphaltic roofing membranes are often coated with granular material. Among the benefits associated with the use of these granules is the ability to reflect and/or block solar radiation, which includes ultraviolet, visible, and infrared radiation, to thereby maintain a cooler roof surface. Other benefits include optional color, weatherability, and fire resistance. It is believed that by increasing the reflectivity of the roofing surface, energy savings can be achieved. There is, therefore, a desire to increase the reflectivity of roofing surfaces, particularly those that are covered with asphaltic membrane. Incremental improvements in reflectivity translate into energy savings for the consumer while also providing environmental benefits.
Conventionally, granules for roofing applications have been comprised of inert mineral particles. U.S. Pat. No. 7,241,500 discloses that “suitably sized particles of naturally occurring materials such as talc, slag, granite, silica sand, greenstone, andesite, porphyry, marble, syenite, rhyolite, diabase, greystone, quartz, slate, trap rock, basalt, and marine shells can be used, as well as recycled manufactured materials such as crushed bricks, concrete, porcelain, fire clay, and the like”.
Typically granules used for roofing applications are of a standard size, and may be classified as full grade or No. 11 grade which, according to U.S. Pat. No. 6,238,794, corresponds to −10/+35 U.S. mesh size. Alternatively, as stated in U.S. Pat. No. 6,933,007, industry standard granules are typically referred to as 8×40 U.S. mesh whereas the average particle size corresponds to about 19 US mesh (i.e. 0.937 mm sieve opening). U.S. Pat. No. 7,241,500 describes the mineral particles, which can be produced by a series of quarrying, crushing, and screening operations, as having a size generally intermediate between sand and gravel, or between about 8 US mesh and 70 US mesh.
U.S. Pat. No. 3,169,075 discloses conventional mineral roofing granules surfaced with a coating comprised of calcined kaolin, a pigment such as titanium dioxide, and an alkali silicate to form a vitreous coating. U.S. Pat. No. 6,933,007 discloses as background that kaolin clay is used extensively in silicate paint formulations for coloring roofing granules, serving as a filler, extender, moisture release agent, and reactant to aid film insolubilization during high temperature firing. However, as further stated in U.S. Pat. No. 6,933,007, kaolin clay alone lacks the brightness and opacity needed to hide the dark underlying base rock of the granule and frequently require appreciable amounts of TiO2 to achieve desired color specifications. U.S. Publication No. 2010/0203336 discloses uncalcined kaolin dispersed with binder and solar reflective particles (such as TiO2) to provide roofing granules which are then cured from about 500 to 800° C. The prior art does not contemplate reflective roofing granules or coatings for granules comprised of additive-free calcined kaolin.